RDS: return to a single-threaded krdsd

We were seeing very nasty bugs due to fundamental assumption the current code
makes about concurrent work struct processing.  The code simpy isn't able to
handle concurrent connection shutdown work function execution today, for
example, which is very much possible once a multi-threaded krdsd was
introduced.  The problem compounds as additional work structs are added to the
mix.

krdsd is no longer perforance critical now that send and receive posting and
FMR flushing are done elsewhere, so the safest fix is to move back to the
single threaded krdsd that the current code was built around.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

net/rds/threads.c

@@ -214,7 +214,7 @@ void rds_threads_exit(void)
 
 int __init rds_threads_init(void)
 {
-	rds_wq = create_workqueue("krdsd");
+	rds_wq = create_singlethread_workqueue("krdsd");
 	if (!rds_wq)
 		return -ENOMEM;